Surface-propagating acoustic wave device

Abstract

In a surface acoustic wave device which includes a relfective multistrip coupler (4, 4'), direct electrical breakthrough resulting from the adjacent location of the input and output transducers (2, 3) is reduced by including at least one forward multistrip coupler (24,25) in each path coupling the transducers (2,3) via the reflective couplers (4,4') so as to space the transducers (2,3) from one another. In a so-called low-loss device in which both ends of the transducers (2,3) are reflectively coupled, the arrangement enables the input/output track separation (23) of the reflective couplers to be reduced thereby significantly reducing the insertion loss of the device.

I claim: 
     
       1. A surface acoustic wave bandpass electrical filter comprising a piezoelectric substrate able to propagate acoustic waves at a surface thereof, input transducing means arranged to launch acoustic wave energy along a propagation track at said surface, output transducing means arranged to receive acoustic wave energy from a further propagation track at said surface which is substantially parallel to but does not overlap said first track, and electrical reflective multistrip coupling means arranged to receive acoustic wave energy from the input transducing means and to relaunch at least part of that energy as acoustic wave energy back towards the output transducing means, said input and output transducers being formed by respective interdigital electrode arrays each comprising parallel strip electrodes which are respectively connected to a corresponding one or other of a pair of bus bars extending along the corresponding sides of the interdigital electrode array, and in which the respective reflective coupling means includes a reflective multistrip coupler comprising a first and second array of mutually parallel conducting strips arranged respectively across an input and an output propagation track, each array including a first set of mutually insulated alternately widely and closely spaced strips, and a second set of electrically interconnected strips arranged so that at least one strip from the second set is situated between substantially every widely spaced pair of strips forming the first set, each strip forming a widely spaced pair in the first set of one array being respectively connected to a corresponding strip forming a closely spaced pair in the first set of the other array and the second set of strips in both arrays being connected together, and at least one forward multistrip coupler included in the surface acoustic wave propagation path and arranged to couple the corresponding ends of the input and output transducers via said reflective multistrip coupler in a manner such that the input and output transducers are spaced apart transversely. 
     
     
       2. A low-loss surface acoustic wave bandpass electrical filter comprising a piezoelectric substrate able to propagate acoustic waves at a surface thereof, input transducing means arranged to launch acoustic wave energy along a propagation track at said surface, output transducing means arranged to receive acoustic wave energy from a further propagation track at said surface which is substantially parallel to but does not overlap said first track, and electical reflective multistrip coupling means respectively arranged to receive acoustic wave energy from each end of the input transducing means and to relaunch at least part of that energy as acoustic wave energy back towards a corresponding end of the output transducing means in a co-operating transductive relationship, said input and output transducers being formed by respective interdigital electrode arrays each comprising parallel strip electrodes which are respectively connected to a corresponding one or other of a pair of bus bars exending along the corresponding sides of the interdigital electrode array, and in which the reflective coupling means are formed by respective reflective multistrip couplers each comprising a first and a second array of mutually parallel conducting strips arranged respectively across an input and an output propagation track, each array including a first set of mutually insulated alternately widely and closely spaced strips, and a second set of electrically interconnected strips arranged so that at least one strip from the second set is situated between substantially every widely spaced pair of strips forming the first set, each strip forming a widely spaced pair in the first set of one array being respectively connected to a corresponding strip forming a closely spaced pair in the first set of the other array and the second set of strips in both arrays being connected together, and at least one forward multistrip coupler included in each surface acoustic wave propagation path and arranged to couple corresponding ends of the input and output transducers via a said reflective multistrip coupler in a manner such that the input and output transducers are spaced apart transversely. 
     
     
       3. A filter as claimed in claim 2, characterised in that a respective forward multistrip coupler is included in the or each said propagation path between the end of each transducer and the corresponding array track of the associated reflective multistrip coupler. 
     
     
       4. A filter as claimed in claim 3, characterised in that the or each surface acoustic wave propagation path which is arranged to couple corresponding ends of the input and output transducers includes an odd plurality of said reflective multistrip couplers. 
     
     
       5. A filter as claimed in claim 2, characterised in that at least one weighted forward multistrip coupler is included in the or each surface acoustic wave propagation path which is arranged to couple corresponding ends of the input and output transducers. 
     
     
       6. A filter as claimed in claim 2, characterised in that at least one of the input and output transducers is in the form of a withdrawal weighted interdigital electrode array. 
     
     
       7. A filter as claimed in claim 2, characterised in that the input and output transducers and the multistrip couplers are arranged to launch, propagate and receive surface acoustic wave energy in the single symmetrical guided surface acoustic wave propagation mode. 
     
     
       8. A filter as claimed in claim 1 wherein a respective forward multistrip coupler is included in a propagation path between the end of each transducer and the corresponding array track of the associated reflective multistrip coupler. 
     
     
       9. A filter as claimed in claim 1 wherein a surface acoustic wave propagation path which is arranged to couple corresponding ends of the input and output transducers includes an odd plurality of said reflective multistrip couplers. 
     
     
       10. A filter as claimed in claim 1 wherein at least one weighted forward multistrip coupler is included in a surface acoustic wave propagation path which is arranged to couple corresponding ends of the input and output transducers. 
     
     
       11. A filter as claimed in claim 1 wherein at least one of the input and output transducers comprises a withdrawal weighted interdigital electrode array. 
     
     
       12. A filter as claimed in claim 1 wherein the input and output transducers and the multistrip couplers are arranged to launch, propagate and receive surface acoustic wave energy in the single symmetrical guided surface acoustic wave propagation mode. 
     
     
       13. A filter as claimed in claim 2 wherein a surface acoustic wave propagation path which is arranged to couple corresponding ends of the input and output transducers includes an odd plurality of said reflective multistrip couplers. 
     
     
       14. A filter as claimed in claim 13 wherein at least one weighted forward multistrip coupler is included in a surface acoustic wave propagation path which is arranged to couple corresponding ends of the input and output transducers. 
     
     
       15. A filter as claimed in claim 9 wherein at least one weighted forward multistrip coupler is included in a surface acoustic wave propagation path which is arranged to couple corresponding ends of the input and output transducers.